A converter is a power processing circuit, that may have an input-output transformer isolation, that operates to convert an input voltage waveform with a DC component into an output DC voltage waveform. The presence of an isolation transformer requires the use of a rectifier circuit in the converter output circuit to perform the waveform conversion. The traditional rectifier uses rectifying diodes that conduct the load current only when forward biased in response to the input waveform. In some rectifiers (i.e. synchronous rectifiers) the diodes are replaced by controlled switches that are periodically biased into conduction and nonconduction in synchronism with the periodic waveform to be rectified. In self-synchronized synchronous rectifiers the biasing of the synchronous switches is supplied directly from a secondary winding of a transformer without requiring a separate drive to activate the synchronous switches.
Self-synchronized synchronous rectifiers come in many forms, all designed to meet specified operating constraints. The challenge, in each instance, is to devise synchronous rectifier circuitry that is efficient (i.e. has low power dissipation) in performing the rectification process. The specific circuit topology of the synchronous rectifier is dependent in large part on the converter type being used and its operating characteristics (i.e. hard switched rs. soft switched). Application of self synchronized synchronous rectifiers to hard switched buck derived converter topologies, for example, is limited by a variable transformer reset voltage that often causes the voltage across the transformer windings to be essentially zero during a portion of each switching cycle. During this time, the synchronous rectifier switch that should be conducting is operating in a dissipative or cut-off mode causing a serious shortfall in efficiency. An example of a circuit that eliminates the problem of zero voltage across the transformer is provided in the U.S. Pat. No. 5,303,138 which discloses an improved forward converter combined with a self synchronized synchronous rectifier. In this circuit the reset voltage is clamped and maintained over the non conducting interval of the main power switch and hence causes the rectifier to operate over the entire non conducting interval. In this arrangement the gate drive signal is directly dependent upon the voltage of the secondary winding which in turn is dependent upon the input voltage and load. In practice the voltages of the secondary winding may vary over a substantial range and there is the possibility of insufficient drive voltage for a rectifier that is conducting, causing it to operate in either a dissipative mode or a cut-off mode. This deficiency is quite likely for converters that deliver low output voltages.
In a circuit disclosed by L. Hubler et al (APEC 94 page 645, entitled "Design of a High Efficiency Power Converter for a Satellite Solid-State Power Amplifier"), the problem of insufficient drive voltage is overcome by including separate windings on the power transformer to drive the synchronous rectifier switches. However, when the turns of the drive windings are set high enough to ensure adequate drive voltage for all operating conditions of input voltage and load, excessive drive voltage is typically generated at some operating condition. This causes excessive power dissipation or failure of the synchronous rectifier switch.
In another U.S. Pat. No. 5,274,543 voltage limiting (gate drive) switches are disclosed as a means for limiting dissipation in the drive circuit for the synchronous rectifiers.